Fabric speaker

The theory behind speaker operation is pretty simple. There’s a coil that is attached to some type of diaphragm and a permanent magnet. When electrical signals pass through the coil a magnetic field is generated, and that field’s interaction with the permanent magnet causes the diaphragm to vibrate and create sound. But we’ve always assumed that the vibrating material must be stretched tight for this to work. [Hannah Perner-Wilson] proved us wrong by making this speaker out of fabric. It uses conductive tape as the coil on a heavy piece of canvas. The permanent magnet is resting on a table and for the demonstration the fabric is just laid on top.

Check out the video after the break to hear the sounds generated by this device as well as a design that uses conductive thread instead of tape. This gets us wondering if what we’re hearing is the result of the magnet vibrating against the tabletop? Let us know your thoughts, and if you’ve got any information about the paper-backed circuit (seen at 0:04 into the video) driving the speakers we’d love to hear about that too.

My instinct tells me that the fabric is just the equivalent of the “former” in a standard speaker. i.e. the bit that gives the coil wire its physical support and geometry to make the voice coil. As far as I can see, in this case the diaphragm (speaker cone equivalent) is the table, actuated by the vibrating magnet….

“But we’ve always assumed that the vibrating material must be stretched tight for this to work.”

About 20 years ago, I would some bell wire around a permanent magnet, hooked it to a headphone output from a tape player, and was able to hear the audio at a reasonable quiet volume. No diaphragm was used, and no parts were easily movable, but the speaker will worked. In my case, I used a permanent magnet that had a copper shell around the back and sides, but any magnet should work.

It was also about 20 years ago or more, when I was a kid I stuck a 500uf capacitor on one of the speaker outputs from home stereo and was able to hear music from it! That was something pretty amusing to me as a kid.

.. so I picked up my mouse and listened, and lo and behold when you hold it against your ear you hear a soft continues beep.
Seems that’s common but I never realized it until one type overdid it and made me listen to a mouse.

Opposing field coils would work, the problem is that they would tend to cancel each others sound waves out unless one is held immobile. Remember that there is a full waveform being reproduced. another problem as stated above would be field density(number and size and spacing of coils).

Interesting concept, but would need lots of improvements to be made into something that would be better than readily available speakers. Now as an actuator of some sort…hmmm

with speakers the efficiency is all about B*l. B=the strength of the magnet and l=the length of the wire.

two opposing magnet wires would work fine, just less efficient. (many motors, including AC fan motors and automobile alternators use this technique… that’s why alternators are tricky to use as bike generators and fans don’t produce electricity on the plug if you spin them- there’s no magnetic field for the coils to move through)
if you have tons of power available, efficiency isn’t much of a problem…

here’s an idea, how about actually sewing the wire into the material into an extremely dense coil? if they can knit reindeer into a sweater they should be able to knit a coil, no?

it’s better to have some kind of rigid body to move air. if you just make fabric move, it may just go floppy. perhaps a thin sheet o magnet, like for homemade fridge magnets would fit into clothing, etc.

or maybe you could exploit the effect of the table, and have a rigid-ish piece of material that the magnet bounces off of to make sound. you want something rigid in there to make it loud.

I think to truly be of service it would useful to know the voltage going into this. 5-100V is vague. power=v*v/r, so with 13 Ohms, you’re sending 25/13~=2 Watts to 10000/13~=7.6k Watts (I’m sure the supply limits the current to way less than 7.6kWatts.)

btw electromagnetic speakers are inherently very inefficient! a typical speaker is 0.5% to 4% efficient! yes, as much as 99.5% of the power is wasted. other types of speaker such as piezo and carbon nanotube are more efficient, but only at frequencies above ~=2kHz.

speakers are rated by efficiency, a.k.a. sensitivity. A typical speaker (with 0.5-4% efficiency) will produce about 90-96dBA SPL at 1 meter. Who knows what SPL Hannah’s device is putting out, but when I get back to the Media Lab on Thursday I can measure it :)

Also, acoustic structures can increase the efficiency of speakers at the cost of bandwidth (in the bass range). e.g. horn structures can make a speaker up to 50-60% efficient if carefully designed. The tradeoff will be that it only reaches that efficiency at a narrow frequency range. Also working against speaker designers is that horns are only practical at a narrow frequency range…at low frequencies they just get too damn big… well i take that back, they can be used in permanent installations.

one final thing… doubling voltage squares the power- which sounds good, but you have to 10x the power in order for it to sound twice as loud :O

@jacksprat yeah a painting might be good because it has a large diaphragm. that’s another factor in speaker efficiency…

C’mon everybody – look up Magnepan Speakers – I have a pair of these in my living room called the SMG-A – they sound awesome, are 4′ x 2′ and an inch thick. This is not novel nor new, or as they say “nothing new under the sun”.